We need all of the ten numbers to solve this (your question only gives six).
Then you can express the given statement into an algebraic equation:
(Sum of the ten numbers)/10 = 8
Multiplying both sides by 10..
=> Sum of the ten numbers = 80
Now put down the ten actual numbers on the left hand side, including (x-3)
2 - 5 - 2 + 5 + (x-3) + 10 .... = 8
Now solve for x